Most current-day, digital-still cameras (DSCs) usually employ a charge-coupled device (CCD) sensor for image capture. These image sensors include a two-dimensional array of photosites. The photosites, or pixels as they are commonly referred to in the art, collect incoming photons and convert them to electron-hole pairs (EHPs). The number of EHPs generated is a linear function of sensor-plane irradiance and a non-linear function of wavelength. Typically, the electrons from these EHPs are collected within the photosites, and subsequently transferred as charge packets within the CCD to an output structure wherein they are converted to a voltage. This voltage signal is detected by off-chip circuitry, which processes these signals and converts them into a digital image. In addition to the signal electrons contained within each charge packet, there is an unavoidable quantity of electrons that get collected as a result of dark-current generation. Since this additional dark-current charge did not result from the incoming image photons, it represents noise, and is hence, undesirable since it reduces the signal-to-noise ratio of the image. Therefore, it is desirable to suppress or eliminate as much of this dark-current charge as possible. There have been many manufacturing and device operational methods employed in the past to reduce the dark signal, as are well known in the art. For example, defect or impurity gettering methods can reduce the generation from depletion-region and/or bulk states, while accumulation-mode clocking is effective at suppressing the generation from surface-states. This is discussed in U.S. Pat. No. 5,115,458.
During normal, single-shot operation of a DSC, this dark current is collected prior to and during image integration, as well as the readout period. Reduction of the dark-current charge that accumulates in the period just prior to image capture, can be accomplished by quickly “flushing” the image area as described by Shepherd, et al. in U.S. Publication No. 2003/0133026. This method basically consists of quickly clocking out the CCD after the shutter button is depressed. The time between when the shutter button is depressed and the shutter actually opens is often referred to as the shutter latency or lag time. Although this prior-art flush method is highly effective, the more pixels the sensor contains, the longer it takes to accomplish. Therefore, as the trend in the industry for more and more pixels continues, the shutter lag starts to become noticeable and objectionable to the photographer. Also, high-speed clocking of the CCDs to flush out the residual dark current in accordance with U.S. Publication No. 2003/0133026 requires a significant amount of power. Therefore, there exists a need in the art to reduce the shutter latency and power consumption.
Consequently, the present invention describes a structure that allows quick and efficient removal of any dark current accumulated within the CCDs just prior to image capture for reduced shutter latency, while reducing power dissipation.